1. Field of the Invention
This invention relates to an optical transceiver.
2. Description of the Related Art
Along with the popularization of a broadband network in recent years, attempts have been made to achieve higher speed, downsizing, and lower costs for an optical transceiver (optical transceiver module) for optical fiber transmission. To achieve the higher speed, at present, a 100-Gbit/s-class optical transceiver has begun to replace a related-art optical transceiver having a bit rate of about 10 Gbit/s. To achieve the downsizing, Multi Source Agreement (MSA) of the Ethernet (trademark) system is the basis and at present, a case volume has been reduced from CFP to CFP2, CFP4, and QSFP28 (MSA standards, respectively) (see http://www.cfp-msa.org/).
According to the MSA standard, serial data of 100 Gbit/s is transmitted as a 4-wavelength multiplexed optical signal through an optical fiber. On the surface of a printed circuit board of the optical transceiver, channels (transmission lines) including digital modulation signal transmission lines are formed. Four channels are formed for transmission, and four channels are formed for reception. A bit rate of an electric signal transmitted through each channel is 25.78 Gbit/s (more specifically, 25.78125 Gbit/s in the case of the method compliant with IEEE 802.3ba), or both 25.78 Gbit/s and 27.95 Gbit/s (more specifically, 27.95249339 Gbit/s in the case of the OTU4 method compliant with ITU-T G.959.1).
In a network device on which the optical transceiver is mounted, it is required to keep intensity of an unnecessary electromagnetic wave generated by the device equal to or less than a limit value defined by the law. For example, in the U.S., a limit value 53.9 dB (μV/m) (in the case of Class B standard, distance 3 m, and frequency range of 1 GHz to 40 GHz), which is defined in FCC Part 15 Subpart B standard, or lower must be satisfied. In a large-sized network device, several to several hundreds of optical transceivers are mounted in slots on the front surface in many cases. In each optical transceiver, an integrated circuit (IC) for driving a digital modulation signal generates a switching noise based on its operation bit rate. This leads to generate an unnecessary electromagnetic wave from the IC or the transmission line. A main component of the unnecessary electromagnetic wave is generated at a frequency based on the operation bit rate. Specifically, when a bit rate is 25.78 Gbit/s, the main component is generated at a frequency of 25.78 GHz. When a bit rate is 27.95 Gbit/s, the main component is generated at a frequency of 27.95 GHz. A design technology for reducing radiation of such an unnecessary electromagnetic wave to the outside of the device is important for both the network device and the optical transceiver.
A size and a rough shape of a case (also referred to as housing, shell, or chassis) of the optical transceiver are defined by each MSA. The case includes a metallic component usually formed by die-casing or sheet-metal processing. Accordingly, a shield effect of the case of the optical transceiver with respect to the electromagnetic wave is relatively high. In reality, however, there may be many cases where the above-mentioned numerical values defined by the law are not satisfied even with the shield effect of the case. In order to prevent any of an unnecessary electromagnetic wave generated in the case from leaking to the outside by achieving the higher shield effect of the case of the optical transceiver, a gap between fitted portions of the case components and a gap to be generated between an optical connector coupled to the optical fiber or an electric connector coupled to the network device and the case component are reduced as much as possible. Specifically, it is ideal to set lengths of all the gaps with respect to a wavelength in air at a frequency of concern to be smaller than a ¼ wavelength, or eliminate the gaps. However, it is difficult to obtain a high shield effect irrespective of manufacturing fluctuations, and achievement thereof is impractical because of an accompanying increase in manufacturing cost.
As another method for reducing the unnecessary electromagnetic wave from the optical transceiver, there is a method of reducing, in the case, the unnecessary electromagnetic wave generated therein. For the purpose of achieving this, for example, a plurality of structures has been provided as described below.
Specifically, Japanese Patent Application Laid-open No. 2009-164308, U.S. Pat. No. 7,195,404, Japanese Patent Application Laid-open No. Hei 04-248703, Japanese Patent No. 4428962, and white paper of Laird HP “Theory and Application of RF/Microwave Absorbers” http://www.eccosorb.com/resource-white-papers.htm, file name: “Absorbers_White_Paper.pdf” concern cavity resonance of the metallic case. When the frequency of the unnecessary electromagnetic wave of concern and an eigenmode frequency of the cavity resonance of the metallic case are equal to or near each other, unnecessary radiation from the optical transceiver may increase. Therefore, according to Japanese Patent Application Laid-open No. 2009-164308, this problem is prevented by employing a structure in which the eigenmode frequency in the metallic case is shifted. According to U.S. Pat. No. 7,195,404, Japanese Patent Application Laid-open No. Hei 04-248703, Japanese Patent No. 4428962, and the white paper of Laird HP “Theory and Application of RF/Microwave Absorbers”, by arranging a wave absorber using a magnetic material or a resistor at a predetermined location in the metallic case to cause a loss in magnetic or electric field, energy of an electromagnetic wave of the cavity resonance is attenuated.
According to U.S. Pat. No. 7,917,037 and Japanese Patent Application Laid-open No. 2008-249856, while there is no mention of cavity resonance, nor reduction thereof is a purpose, a wave absorber is arranged at a predetermined location inside the case of the optical transceiver. Specifically, according to U.S. Pat. No. 7,917,037, the unnecessary electromagnetic wave is attenuated by arranging nonconductive (or quasi-nonconductive) wave absorbers at two upper and lower locations of the printed circuit board and placing the wave absorbers at the locations nearer to an electromagnetic interference (EMI) source (excitation source). The wave absorbers are not fixed to the case. According to Japanese Patent Application Laid-open No. 2008-249856, by arranging wave absorbers using magnetic materials to serve as shields between two printed circuit boards and upper and lower cases, radiation from edges of the boards is attenuated. Further, the wave absorbers are arranged to be bonded to top and bottom surfaces without any space in the case. No wave absorber is arranged in an intermediate region of the two printed boards.